Stereophonic transmission system



Feb. 17, 1959` A. DAUGUET 2,874,221

l I rS'IEREC)PHONlC TRANSMISSION SYSTEM Filed Jan. l5, 1956 E'. vE2

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C |4||||| ll' /S AMPL/F/ER E ALEXANDRE DAUGUET www AGENT United States Patent O srEREorHoNrc TRANsMlssIoN SYSTEM Alexandre Dauguet, Paris, France, asslgnor, by mesne assignments, to North American Philips Company, Inc.,

New York, N. Y., a corporation of Delaware Application January 13, 1956, Serial No. 559,017

Claims priority, application France January 17, 1955 aclaims. (ci. 11s-1s) The invention relates to stereophonic transmission Vsystems and to transmitters and receivers for use therein, which transmission systems canbe used advantageously for sound recording, transmission by conductors, Wireless transmission with the use of amplitude modulation or frequency modulation and the like.

In this system, the transmission device-is provided with two transmission channels connectedtoa common output circuit, the iirsttransmission channel-'comprising a summation device and the second channel comprising a subtraction device. At the receiver end, the signals which sion of the stereophonic signals is reduced.

The system in accordance with the invention is characterized in that, at the transmitter-end, the sum signal is supplied directly to the common output circuit, whereas. the difference signal is supplied to this output circuit through the cascade arrangement of a iilter which suppresses the highest signal frequencies and of a single-sideband modulator having an auxiliary carrier-wave oscillator connected to it which transposes the difference signal with reduced band-width to a frequency-band lying above the highest signal frequency to be transmitted, whilst, at the receiver-end, the common input circuit is connected to the two reception channels by way of separating lters, the sum signal occurring in the lrst reception channel being supplied directly to a summationand subtraction device connected in the output circuit of both channels, to which device the single-sideband difference signal modulate on the auxiliary carrier-wave frequency occurring in thesecond reception channel is supplied through a demodulator stage.

In the system in accordance with the invention, the difference signal can be transmitted with a considerably reduced band width, for example about 2000 C./S., which permits the use of a comparatively small additional band width for the stereophonic transmission. In spite of the highly limited band width of the difference signal, which signal consequently gives an unpleasant sound impression per se, the stereophonic sound image reproduced at the receiver end proves to be very satisfactory.

In order to demodulate the single-sideband difference signal by which the auxiliary carrier-Wave frequency is modulated, at the receiver-end the auxiliary carrier-wave frequency can be recovered and be supplied to the demodulator stage for demodulation of the single-sideband signal. As an alternative, the auxiliary carrier-wave frequency may be transmitted together with the single-side- 2,874,221V Patented F eb. 17,v 1959 2 band difference signal by which it is modulated in which event the demodulation can be effected by means of an amplitude detector or a quadratic detector.

The invention and its advantages will now be described more fully with reference to the figures.

Fig. 1 shows a stereophonic transmission arrangement in accordance with the invention.

Fig. 2 is a frequency diagram of the frequency-band of the signals transmitted by the transmission arrangement shown in Fig. 1, whilst Fig. 3 shows a stereophonic reception arrangement in accordance with the invention. Y Fig. 1 shows a stereophonic transmitter in accordance with the invention, in which the ,associated stereophonic signals E1 and E2 are taken from two microphones C1 and C2, which microphones may, for example, be secured to an artificial head. The signals E1 and E2 from the microphones C1 and C2 are supplied, by way of a control-desk P, to a summation device and a subtraction device constituted by mixer amplifiers A1 and A2 respectively, the `output signals of which are represented by E1+E2 and E1-E2, respectively.

A low-pass filter F2 having a cut-oi frequency of 10,000 C./S. is connected to the output of the mixer amplier A1, so that the sum signal E1+'E2=2 is transmitted with a suiciently large band width to ensure a satisfactory quality of reproduction, whereas a low-pass filter F1 having a considerably smaller cut-off frequency of, for example, about 2000 C./S., being connected to the output of the mixer amplifier A2. The response characteristics of the filters F1 and F2 are shown in Fig. 2 by the curves r1 and r2 respectively, the curve r1, as shown, having been shifted to a frequency range lying between -11,000 and 13,000 C./S.

In the arrangement shown, the difference' signal E11-E2 derived from the low-pass iilter F1 is supplied to a singlel sideband modulator M1 having a local oscillator O connected to it, which transposes the diierence signalof reduced bandwidth to a frequency band lying above-the highest signal frequency, which auxiliary carrier-wave frequency, may, for example, be 11,000 C./S. The auxiliary carrier-wave frequency of 11,000 C./S. is transmitted together with the transposed difference signal A' of reduced band width. The transposed difference signal is shown by the curve r1 in Fig. 2.

In the arrangement described, the transposed diiierence signal A is added in a mixer amplitier M2 to the sum signal 2 derived from the iilter F2 and supplied as a modulation voltage to an amplitude modulator stage T which is connected to an aerial. The frequency band to be transmitted is shown in the frequency diagram of Fig. 2 and, as will be seen from this gure, the band width of the transmitted stereophonic signals extends only to about 13,000 or 14,000 C./S.

At this point it should be mentioned that the signals emitted by the transmitter described can be listened to substantially without disturbances by means of a normal amplitude modulation receiver having the usual pass band lextending up to about 9000 C./S. to 10,000 C./ S. Obviously, in this event no stereophonic reproduction is obtained.

Fig. 3 shows a stereophonic receiver in accordance with the invention, in which the stereophonic signals which are received by way of an aerial, after eventually high-fre quency amplification, intermediate-frequency amplification and detection in a stage R are separated from one another by means of separating lters in the form of a low-pass filter F3 having a cut-off frequency of 10,000 C./S. and of a band-filter F4 having a pass-band between 10,000 C./S. and 14,000 C./ S. The lowpass iilter F2 passes the sum signal which lies in the frequency-band from 0 to 10,000 C./,S., whilst the band-lter F4 passes the auxiliary f 3 carrier-wave frequency of 11,000 C./ S. and the single-sideband dilerence signal by'which it is modulated.` In vorder to recover the difference signal, the single-sideband dilerence signal is demodulated in a quadratic detector D the output circuit of which contains'a low-pass iilter'F having a cut-off frequency of 2000 C./ S. A i

In the arrangement shown, the sum signal derived from the lter F3 and the diierenee signal derivedl from the filter F5 are supplied to amplifiers A3 and A., respectively, which amplifiers also act as a summation device vand a subtraction device by coupling their output circuits in the manner shown in the ligure. The output signals obtained by summation and subtractionV are supplied to separate reproduction devices HP1 and HP2.

Thus, each of the loudspeakers HP1 and HP2 `reproduces the signals lying between 0 and 10,000 YCQ/S., the signal frequencies up to about 2000 CJS. being reproduced stereophonically. The stereophonic sound image reproduced by the reproduction devices HP1 andHP: proves to be of veryhigh quality.Y

It should be mentioned here that the summation and subtraction of the stereophonic signals can be effected by means of transformers or resistors instead of by amplifiers.

What is claimed is:

1. A stereophonic transmission system comprising a transmitter and a receiver, said transmitter comprising a rst channel having a signal summation circuit, a second channel having a signal subtracting circuit, two sources of signals, means connected toapply both of said signals to both of said channels, means connected in said second channel to limit the bandwidth of the difference signal, a

' :gemaal single-sideband modulator connected in said second channel to transpose the limited difference signal to a frequency range which is higher than the highest frequency of the signal in said first channel, common output means connected to combine the output signals of said first and sccond channels, and said receiver comprising trst and second reception channels, means connected to apply the received signal corresponding to the output of said Vfirst transmitter channel to said first reception channel, means connected to apply the received signal corresponding to the output of said second transmitter channel to said second reception channel, a demodulator stage connected in said second reception channel to transpose the difference signal to its original frequency range, a pair of reproducing devices, and summation and subtraction circuits connected between the outputs of said rst and second reception channels and said reproducing devices.

2. A system as claimed in claim l, in which said means to limit the bandwidth of thc dierence signal comprises a l0w-pass filter' having a bandwidth narrower than the bandwidth of said signal, and including a corresponding low-pass filter connected in said second reception channel to limit the bandwidth of the demodulated signal.

References Cited in the file of this patent UNITED STATES PATENTS 1,685,357 Griggs sept. 25, 192s 2,536,664 Sinnett et al Jan. 2, 1951 2,698,379 Boelens et al Dec. 28, 1954 '2,779,020 Wilmotte Jan. 22, 1957 

